1. Field of the Invention
The present invention relates to a positive temperature coefficient semiconductor ceramic, and more particularly, to a positive temperature coefficient semiconductor ceramic in which low resistance and excellent resistance/temperature characteristics are obtained with good reproducibility, and which also exhibits a large withstand voltage. 2. Description of the Related Art
It is known that a positive temperature coefficient semiconductor ceramic may be produced by adding a trace amount of one of the rare-earth elements, Nb, Ta, Bi, Sb or W to a barium titanate family ceramic.
It is also known that the rate at which the resistance of the positive temperature coefficient semiconductor ceramic changes with temperature may be increased by adding manganese Mn to such a positive temperature coefficient semiconductor ceramic. Addition of Mn, however, causes the resistivity of the resultant positive temperature coefficient semiconductor ceramic to increase. To prevent this, it is widely practiced to add silicon dioxide SiO.sub.2 to the positive temperature coefficient semiconductor ceramic together with Mn.
Further, it is also known that the withstand voltage may be improved by substituting part of the Ba contained in the barium titanate semiconductor ceramic with calcium Ca.
However, the amount of SiO.sub.2 that can be added is limited to between 0.2 and 3.5 mole %, because 0.2 mole % SiO.sub.2 or less is not enough to reduce the resistivity and 3.5 mole % SiO.sub.2 or more makes the resultant positive temperature coefficient semiconductor ceramic vitreous and increases the resistivity thereof. These factors in turn limit the amounts of elements that can be added to the ceramic in efforts to improve the characteristics, such as Mn which is used to improve the resistance/temperature characteristics or Ca which is added to improve the withstand voltage.
More specifically, though the characteristics of the ceramic may be improved by the addition of the elements that are effective to improve the characteristics, such as Mn or Ca, addition of these elements causes the resistivity of the resultant semiconductor ceramic to increase. To prevent this, SiO.sub.2 may be added to the ceramic together with any of these elements. However, the addition of SiO.sub.2 in an amount of 3.5 mole % or more makes the resultant semiconductor ceramic vitreores, as well as increases the resistivity thereof. Thus, it is only possible to add the elements used to improve the characteristics such as Mn and Ca in amounts that correspond to the content of SiO.sub.2 which is effective in reducing the resistivity.